Blender Git Loki

RNA: add option to enable by default lib overridale flag of defined properties.

Similar to the one allowing to deactivate DNA check, etc.

Will helps reduce verbosity when making many new properties overridable.

Note that pointer properties always remain non-overridable by default,
since basically only ID pointers should be.

Reviewed By: brecht

Maniphest Tasks: T77083

Differential Revision: https://developer.blender.org/D7906